Filter narrowing effects were rarely considered as a design issue in optical transmission systems such as WDM systems, since the usual filter bandwidth was always much larger than the bandwidth of a signal bit rate.
A new problem discussed in the present application has occurred, for example, in modern optical systems. Deployment of 40 Gb/s systems is constrained by multiple physical effects and design considerations. The system reach can be improved using advanced modulation formats to increase the tolerance to noise and non-linear effects, but at the same time, the bandwidth requirements of these modulation formats may increase. As bandwidth requirements increase, the system design considerations may limit the use of denser channel spacing and of filter cascades. Furthermore, it is highly desired, especially in 40 Gb/s systems, to use filter-based dispersion compensation modules DCMs (Bragg gratings, etalon, virtual imaged phase-arrays, etc.) rather than dispersion compensation fibers (DCFs) since the DCMs exhibit lower insertion loss and reduce non-linear effects. For flexibility of the network, it is also desired to use reconfigurable-add/drop modules (ROADMs), which also limit the system bandwidth. Multiple cascades of DCMs and ROADMs along with the desired move from 100 GHz to 50 GHz channel spacing create a considerable design constraint on high bandwidth transmission systems.
In view of the above, the combination of the higher bandwidth transmission, especially at or beyond 40 Gb/s, and the increasing use of filtering chains in the optical transmission systems leads to a new problem in designing of optical systems, not yet considered in the prior art.
A usual accepted way for estimating workability of communication systems is testing at least portions of the systems in a laboratory and based on that, making some design solutions. Another widely known approach is performing a mathematical simulation of the communication system, by creating a mathematical model of the system and further is simulating operation of the system by changing parameters of the mathematical model.
The first approach suffers, from practical reasons, by the fact that real communication systems may comprise a large number of elements (for example, filters, amplifiers, Dispersion Compensating Modules DCMs and fibers). Therefore, chaining such elements in a laboratory is impractical due to the high cost of the optical components and the increased time-to-deployment. The second approach requires a complex work of creating and checking the mathematical model. Nevertheless, in many cases the simulation does not give sufficiently precise results.